kubectl-cnpg: Enable auto-completion as kubectl plugin

[Skaronator]

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I am not sure if this is the correct homebrew way but to get the auto-completion working as kubectl plugin we need one more binary called kubectl_complete-cnpg

The whole thing is documented here: https://github.com/cloudnative-pg/cloudnative-pg/blob/main/docs/src/kubectl-plugin.md#configuring-auto-completion

[carlocab]

Thanks!

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